These filters do not directly know whether the API they are using will
support dynamic frame pools, so this is somewhat tricky.  If the user
sets extra_hw_frames, we assume that they are aware of the problem and
set a fixed size based on that.  If not, most cases use dynamic sizing
just like they did previously.  The hardware-reverse-mapping case for
hwmap previously had a large fixed size (64) here, primarily as a hack
for QSV use - this is removed and extra_hw_frames will need to be set
for QSV to work since it requires fixed-size pools (as the other cases
do, and which didn't work before).